1. Field of the Invention
The present invention relates to a fixing apparatus used in an image forming apparatus such as a copying machine or a laser beam printer.
2. Description of the Related Art
An image forming apparatus such as an electrophotographic copying machine or a laser beam printer includes a fixing apparatus that fixes a toner image formed on a recording material by performing heating and pressing. The fixing apparatus is heated by employing a heat roller method that uses a cylindrical member which internally includes a halogen heater. Further, there recently is a film heating method which can save power in the fixing apparatus.
The fixing apparatus employing the film heating method includes a sliding nip portion (hereinafter referred to as an inner surface nip portion or a heat transfer nip portion) and a pressing nip portion (hereinafter referred to as a fixing nip portion). The sliding nip portion is formed by a cylindrical belt (hereinafter referred to as a fixing sleeve) that is based on a heat-resistant resin or metal, and a heating member that is made of ceramic or the like, and contact-slides to an inner surface of the fixing sleeve. The pressing nip portion is formed by a pressing member pressing via the fixing sleeve. The recording material carrying the toner image is held between and conveyed through the fixing nip portion, so that the toner image is fixed on the recording material. Since the fixing apparatus employing the film heating method intensively heats an area surrounding the fixing nip portion, power can be saved and wait time can be shortened (i.e., quick start) as compared to a fixing apparatus employing the heat roller method.
Further, there are various types of recording materials used in the image forming apparatus. Such recording materials to be used in the image forming apparatus include plain paper commonly used in an office, a medium in which a sheet is in folds, e.g., an envelope, and thin paper whose basis weight is 60 g/m2 or less.
Japanese Patent Application Laid-Open No. 2007-128037 discusses a fixing apparatus in which a pressing force can be changed to correspond to various media such as an envelope or thin paper. However, if the pressing force is changed according to the type of media in the fixing apparatus employing the film heating method and including the fixing sleeve formed of a highly rigid member such as metal, a region width of the fixing nip portion becomes narrow when the pressing force is reduced. As a result, heat-transfer efficiency of the heating member becomes lowered due to narrowing of the region width in the inner surface nip portion. In other words, if the pressing force of the fixing apparatus employing the film heating method is reduced to prevent wrinkles to be generated in the envelope or prevent curling, the heat-transfer efficiency when fixing the image on the medium may be lowered.